Now we are getting technical when most of the guns talked of on these pages were made the English Trade worked in "attals" this is an obscure measurment system adopted by many trades and relied on the skill of the crafts man involved.A mixtue of basic measuring equipment and a "feel" for what was right. This was in the days before common use of micrometers and other high tech equipment.
The "attal"in fact was thus .That'al be right and That'al be good enough.